Shaped articles of aluminum nitride, especially fibers thereof, are sought because they possess high thermal conductivity; hence, are useful in electronic applications where heat dissipation is necessary. Thermal conductivity may be an order of magnitude higher than that of alumina but depends critically on material purity. These considerations are discussed in great detail in, e.g., U.S. Pat. No.4,578,365, wherein the preparation of bulk ceramic articles of aluminum nitride is discussed.
Fibers composed nominally of aluminum nitride may be prepared by a number of prior art methods. U.S. Pat. Nos. 3,529,044; 3,653,830; 3,846,527; 4,010,233; 4,126,652; Can. No. 839,321; G.B. No. 1,514,171 and Jap. Appl'n. No. 61/124,626 are illustrative. In one reaction utilizing inexpensive raw materials, alumina and carbon, or an organic precursor thereof, are heated at elevated temperatures in the presence of nitrogen, whereupon aluminum nitride is formed. This is a classic text-book reaction and is described in detail in E.P. No. 0213 629. However, this reaction requires careful stoichiometric control and often results in excess carbon or unconverted alumina, either of which impacts dramatically on thermal conductivity. In fiber form, incomplete sintering (densification) or excessive grain growth can also result, either of which results in inferior fibers. Accordingly, it is the object of this invention to produce aluminum nitride articles, especially fibers, by the in-situ carbothermal reductive nitridation of alumina in a manner permitting optimization of purity and microstructure, to provide thereby strong fibers which are highly conductive of heat.